The Cow - 2:24
If you cannot do that, and you will never be able to do that, then beware of the Fire prepared for the disbelievers, whose fuel is mankind and stones.
Related:
Prophet Jonah/10:38
"Do they say: "He fabricated it"? Say: "If you are truthful, bring a surah like it, and call upon those whom you can call upon besides Allah."
Prophet Hud/11:13
"Do they say: "He fabricated it (the Qur´an)"? Say: "Come on, bring ten suras like it, fabricated. And call all your friends besides Allah, if you are truthful!"
The Mountain/52:33
"Or do they say, "He invented it"? No, they do not believe."
The Mountain/52:34
Let them bring a hadith like it if they are truthful.
The Night Journey/17:88
"Say: "Even if all mankind and jinn gathered together to create the like of this Qur´an and supported each other in it, they could not create the like of it."